Relative to the trajectory of motion, cross flow wind components can be used to generate forward thrust in an airfoil. It is the principle used in sailboats, which are pushed against the wind, when a certain minimum angle is formed between the wind direction and the sailboat trajectory.
In the 1970's the NASA's scientist Richard Whitcomb developed the concept of ‘winglets’ which are now widely employed in aircraft designs. They are based in the same principle of the sailboats. Relative to the aircraft's wing, the winglets are smaller airfoils which are placed on the wingtips in nearly vertical position. The winglets take advantage of the cross flow originated at the wingtips to generate forward thrust forces.
On the other side, single disk propellers generate a significant swirl component in the downstream flow, the resulting rotation angle, ω, being a function of the propeller thrust coefficient. As a rough order of magnitude, ω varies between 20 degrees at take-off power to about 5 degrees for cruise conditions.
The energy associated to the swirl of the propeller downstream flow represents a loss of propulsive efficiency. It can be partially recovered by stators or vanes installed behind the propeller, as it is the case of fans contained in nacelles of jet engines.
Another way of recovering the propeller swirl energy is by a proper integration of the propulsion system onto the vehicle airframe, as is described hereafter with reference to the embodiments of the invention.